Generally, output capacitors with large capacitance are needed in drivers which provide energy to a DC load, so as to absorb the AC portion of the output power. However, the output capacitors with large capacitance are big and high-cost, and the capacitor type is limited. This problem is more critical in LED drivers.
LED has been quickly adopted as a new lighting source because of its high efficiency and environment friendly. Unlike the conventional incandescent lamp, LED cannot be directly driven by the AC line. A driver is needed to convert the AC input into a DC current source. In addition, any LED driver with power rating higher than 5 W needs to satisfy the Power Factor (PF) and Total Harmonic Distortion (THD) requirements. As a result, passive Power Factor Correction (PFC) circuits or active PFC technology is used in these LED drivers.
FIG. 1 is a working waveform of a LED driver with PFC function, wherein vin is the input voltage, iin is the input current, pin is the input power and vo is the output voltage. As long as the PFC is used, the input power doesn't meet the instantaneous power. As a result, an output capacitor with large capacitance is needed to store the energy and reduce the ripple. Normally, an electrolytic capacitor is used because of its large capacitance and high voltage rating. Other types of capacitor can hardly meet both high voltage and large capacitance requirements.
However, a big shortcoming of the electrolytic capacitor is its short life time. It can hardly achieve over 10,000 hours. Comparing with the average life time of LED (>50,000 hours), the electrolytic capacitor has been a major barrier to achieve a long life time of the whole lamp fixture.